I've unlocked only one character so far, and I've been playing for a couple of days. It's a tricky process, and I really don't have any on-hand info on the correct ways to unlock the characters. However if you have the new update, go into stats, and view the area showing the characters and your unlocked weapons you can view the shadows of other characters.
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user18241Jan 13 '12 at 13:01

For the game to tell you how many crates you have to obtain to unlock the next character, you have to have completed the level-unlocking goal first. For example, in this screenshot, the Score 30 to unlock a new character text appears because I have already scored 20 to unlock the Moon Temple (the next stage).